The present invention pertains to a gas separation and purification process and an apparatus therefor, more specifically to a process and an apparatus for recovering xenon.
Demand for xenon is growing based on emerging applications in the manufacturing and healthcare industries.
Xenon is now used in semiconductor related manufacturing processes, such as etching and lithography. It is an attractive additive gas for plasma-based processes because of its size (xenon is one of the largest rare gases), as well as the number of valence level electrons. For example, the addition of xenon can help moderate the electron temperature of plasmas, which can assist with developing new reaction pathways.
Xenon also finds increasing use in the healthcare industry as an anesthetic gas (see, e.g., U.S. Pat. No. 6,236,041 to Donnerhack et al.) and in medical imaging (see, e.g., U.S. Pat. No. 6,408,849 to Spiegelman et al.).
Xenon is also a byproduct from etching processes that employ XeF2. This molecule decomposes on the surfaces of materials such as silicon to release atomic fluorine. The xenon then departs from the surface back into the gas phase.
However, a potential barrier to increased xenon utilization is the relatively high cost of xenon. Xenon exists within the atmosphere at a concentration of only 80 ppb. Thus, the industry has sought to provide methods that extract xenon from air at a lower cost, as well as processes that can reclaim xenon from process effluents before re-introduction to the atmosphere.
U.S. Patent Application Publication No. 2003/0000385 A1 to Kawai et al. discloses a gas separation process and apparatus for obtaining purified gases, such as krypton and xenon. The process comprises a combination of: (1) an equilibrium pressure swing adsorption process for separating gas components based on the difference in equilibrium adsorption; and (2) a rate-dependent pressure swing adsorption process for separating the gas components based on the difference in adsorption rates.
In the healthcare context, the attractiveness of using xenon as an anesthetic has been enhanced by methods of recycling xenon from exhaled gases for further use. For example, U.S. Pat. No. 5,520,169 to Georgieff et al. discloses a method comprising cleaning, compressing and cooling exhaled gas so as to selectively liquefy xenon. The other components of the exhaled composition remain gaseous and are separated from the liquid xenon. Separation based on adsorption is not disclosed.
U.S. Pat. No. 6,134,914 to Eschwey et al. also relates to recycling xenon from exhaled gases. This patent finds fault with the high complexity of devices according to Georgieff et al., as well as the degree of transfer in xenon recovery. Eschwey et al. also teaches a method based on differential phase change separation, but claims to improve upon Georgieff et al. by condensing the exhaled gases under a pressure from 0.6 bar to 150 bar, reducing the purity requirements for the xenon, and compensating for any additional residual fractions of oxygen in the recovered xenon by appropriately adjusting the proportion of oxygen added to the recovered xenon when the anesthetic gas is remixed. Separation based on adsorption is not disclosed.
WO 03/092778 A1 to Taveira et al. also discloses means for recycling xenon from exhaled gases. The exhaled anesthetic gases are first conveyed through a phase change filtration system to remove bacteria and reduce the amount of water vapor, volatile organics and fluorine anesthetics in the mixture. The resulting gas mixture is then passed through a column packed with an adsorbent, such as zeolite 5A, that selectively adsorbs more xenon than oxygen and nitrogen. The xenon-rich gas mixture from the column is then further purified by vacuum swing adsorption using a carbon molecular sieve as the adsorbent.
U.S. Pat. No. 4,674,099 to Turner discloses a method and apparatus for recycling rare gases, such as xenon, from a spent lasing gas mixture from an excimer laser. The rare gases are recovered by passing the gas mixture through a cleaning element comprising in series a hot metal reactor, a heat exchanger and a cryotrap-filter combination.
U.S. Pat. No. 6,658,894 to Golden et al. discloses a method and an apparatus for recovering xenon and/or krypton from an oxygen containing gas, such as liquid oxygen bottoms in a cryogenic air separation plant, by selective adsorption on a Li and Ag exchanged zeolite containing 5 to 40% Ag exchange capacity on an equivalents basis, with periodic thermal regeneration of the adsorbent.
Despite the foregoing developments, it is still desired to provide additional and improved means to obtain xenon from gaseous mixtures.
All references cited herein are incorporated herein by reference in their entireties.